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  <title>自然对数</title>
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  <div id="layout-content"><div id="function.log" class="refentry">
 <div class="refnamediv">
  <h1 class="refname">log</h1>
  <p class="verinfo">(PHP 4, PHP 5, PHP 7)</p><p class="refpurpose"><span class="refname">log</span> &mdash; <span class="dc-title">自然对数</span></p>

 </div>
 <div class="refsect1 description" id="refsect1-function.log-description">
  <h3 class="title">说明</h3>
   <div class="methodsynopsis dc-description">
    <span class="methodname"><strong>log</strong></span>
     ( <span class="methodparam"><span class="type">float</span> <code class="parameter">$arg</code></span>
    [, <span class="methodparam"><span class="type">float</span> <code class="parameter">$base</code><span class="initializer"> = M_E</span></span>
   ] ) : <span class="type">float</span></div>

   <p class="para rdfs-comment">
    如果指定了可选的参数 <code class="parameter">base</code>，<span class="function"><strong>log()</strong></span>
    返回 log<sub class="subscript">base</sub>
    <code class="parameter">arg</code>，否则 <span class="function"><strong>log()</strong></span> 
    返回参数 <code class="parameter">arg</code> 的自然对数。
   </p>
  </div>

  <div class="refsect1 parameters" id="refsect1-function.log-parameters">
   <h3 class="title">参数</h3>
   <p class="para">
    <dl>

     
      <dt>
<code class="parameter">arg</code></dt>

      <dd>

       <p class="para">
        要计算对数的值
       </p>
      </dd>

     
     
      <dt>
<code class="parameter">base</code></dt>

      <dd>

       <p class="para">
        The optional logarithmic base to use 
        (defaults to &#039;e&#039; and so to the natural logarithm).
       </p>
      </dd>

     
    </dl>

   </p>
  </div>

  <div class="refsect1 returnvalues" id="refsect1-function.log-returnvalues">
   <h3 class="title">返回值</h3>
   <p class="para">
    返回 log<sub class="subscript">base</sub>
    <code class="parameter">arg</code>，或者它的自然对数。
   </p>
  </div>

  <div class="refsect1 changelog" id="refsect1-function.log-changelog">
   <h3 class="title">更新日志</h3>
   <p class="para">
    <table class="doctable informaltable">
     
      <thead>
       <tr>
        <th>版本</th>
        <th>说明</th>
       </tr>

      </thead>

      <tbody class="tbody">
       <tr>
        <td>4.3.0</td>
        <td>
         可选参数  <code class="parameter">base</code>可用。
         你可以计算任意以 <em>b</em> 为底 <em>n</em>
      的对数，但其实使用的是数学等式：log<sub class="subscript">b</sub>(n) =
      log(n)/log(b)，其中 log 是自然对数。
        </td>
       </tr>

      </tbody>
     
    </table>

   </p>
  </div>

  <div class="refsect1 seealso" id="refsect1-function.log-seealso">
   <h3 class="title">参见</h3>
   <p class="para">
    <ul class="simplelist">
     <li class="member"><span class="function"><a href="log10.html" class="function" rel="rdfs-seeAlso">log10()</a> - 以 10 为底的对数</span></li>
     <li class="member"><span class="function"><a href="exp.html" class="function" rel="rdfs-seeAlso">exp()</a> - 计算 e 的指数</span></li>
     <li class="member"><span class="function"><a href="pow.html" class="function" rel="rdfs-seeAlso">pow()</a> - 指数表达式</span></li>
    </ul>
   </p>
  </div>

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